In aircraft applications it is highly desirable to minimize the weight of aircraft components as every pound saved in aircraft weight translates to fuel savings and/or greater payload capacity. With respect to propeller, turboprop or turbofan aircraft engine components, it is well appreciated that the propulsor blades are the most likely candidate for weight reduction since the weights of other related components, e.g. blade retention means, pitch change mechanisms, hub disks, shafts and bearings, are typically directly dependent upon the magnitude of the blade centrifugal loading borne by these components. The propulsor blades per se, however, can be made lighter in weight so long as the centrifugal pull, bending moments, torsion loads and vibratory loads, imposed upon the blades during operation are effectively transmitted to the blade retention means for distribution to the aforenoted load bearing components.
It is known in the art to produce lighter weight propulsor blades of a built-up construction wherein a blade is formed of an outer shell made of lightweight composite material, and an internal load bearing spar which is bonded to the interior surface of the shell and extends from within the shell cavity to terminate beyond the shell in a root end which is adapted to be mounted to a suitable blade retention means. Examples of such composite blades are presented in U.S. Pat. Nos. 3,799,701; 4,784,575 and 4,810,167.
It has become conventional practice in the aircraft industry to manufacture such blades with a shell formed about the load bearing spar as a molded fiber reinforced resin body using resin transfer molding methods. Such fiber reinforced resin shells exhibit high strength and low weight characteristics and in aircraft applications typically offer at least as high strength as corresponding articles made of metal at a substantially lower weight. For, example, commonly assigned U.S. Pat. No. 4,648,921 discloses a method of making a fiber reinforced airfoil shaped propeller blade assembly wherein 4 to 7 layers of woven fiberglass cloth are layed up over a foam underbody which is formed by injecting a lightweight foam material into a mold disposed about an adhesive coated full length metallic spar. After curing, the molded underbody is wrapped in multiple layers of the fibrous reinforcing fiberglass cloth, each of the fiberglass layers being trimmed to its desired contour and then hand stitched, a labor intensive practice, in place over the foam underbody. This subassembly is then placed in a second mold and a synthetic polymeric material such as epoxy resin is injected into the fiber matrix and then cured. Alternatively, the resin may be applied to the fibrous cloth of the wrapped subassembly before it is placed into the curing mold.
As discussed in commonly assigned U.S. Pat. No. 4,470,862, the hand stitching may be eliminated by adhesively bonding each fiberglass layer to the layer therebeneath. To do so, the fiberglass material is provided on its underside with a minute, but effective, amount of thermoplastic adhesive. The material is then trimmed to shape and placed in position over the subassembly. Thereafter the adhesive is activated by heat and pressure by means of an electric resistance heated hand iron applied to the surface of the fiberglass material. Although the use of such adhesive coated fiberglass material does indeed eliminate the need for hand stitching, this method of laying up the fiberglass layers is still quite labor intensive and a seam must still be formed.
A light-weight rotary machine blade comprising a composite spar having a metal root and a surrounding fiber-reinforced composite shell is disclosed in commonly assigned, co-pending application Ser. No. 07/633,566, filed Dec. 24, 1990, of John A. Violette and Charles E. K. Carlson, the entire disclosure of which is hereby incorporated by reference. Also disclosed therein is a method for manufacturing such a composite blade comprising the steps of: installing an elongated core of lightweight cellular foam material into to a receiving cavity defined by the flared distal end of a foreshortened metal spar so as to extend axially outwardly therefrom, thence laying up a laminate fiber wrap of alternating plies of spanwisely oriented graphite fibers and angularly woven plies of high strength aramid fibers about this spar subassembly to form a preliminary composite assembly, thence attaching leading and trailing edge fillers of lightweight foam material to the preliminary composite assembly to form the desired contoured shape of the blade, thereafter laying up a laminate wrap of layered plies of high strength aramid fibers about this entire shaped subassembly except for the root end of the spar, and thence placing the wrapped shaped subassembly into a conforming mold and impregnating the wrapped shaped subassembly with an epoxy resin via resin transfer molding techniques to yield an resin reinforced assembly which upon curing constitutes the lightweight composite blade. Additionally, U.S. Pat. No. 4,524,499 discloses a method of manufacturing a composite propeller blade utilizing a resin impregnated fabric wrap over a composite foam spar having a metal root, and U.S. Pat. Nos. 4,268,571 and 4,471,020 each disclose a method of manufacturing a composite propeller blade of spar and shell construction using dry fabric layup of the shell over a preformed spar subassembly followed by resin injection and curing in a mold.